In the discontinuous polymerization of vinyl chloride by aqueous emulsion polymerization, it was originally necessary, as in other vinyl chloride polymerizations, to open the polymerization reactor for cleaning purposes between the polymerization cycles. Although the vinyl chloride content in the interior of the reactor was reduced to a minimum before opening the reactor by evacuation and purging with nitrogen, it was impossible to prevent comparatively large amounts of monomeric vinyl chloride from being emitted into the atmosphere during this operation.
By the introduction of the automatic hydrodynamic cleaning method, as described for example in Austrian Pat. No. 305,724, the opening of the reactor for cleaning purposes between polymerizations is no longer necessary. A cleaning device, operated by the hydrodynamic principle and designed to be pressure-proof, is connected with the reactor, for example above the reflux condenser, and this device is introduced into the reactor, as necessary, between the polymerization cycles. In this way, a manual cleaning operation has become superfluous.
In spite of the use of this hydrodynamic cleaning method, however, the emission into the atmosphere of monomeric vinyl chloride from the gas phase of the reactor between polymerization cycles in large-scale reactors, e.g., of a volume of 60 m.sup.3 and above, was unavoidable.
In modern processes, high-percentage dispersions, i.e., of at least 48% by weight of monomer or solids content, are utilized for reasons of energy savings during the spray-drying and for a better utilization of the manufacturing units. In the formation of these dispersions, it is desirable to employ a minimum amount of emulsifier, since high emulsifier contents have a very deleterious effect in the final products and during the processing operation.
Therefore, modern processes operate, with monomer or solids contents of at least 48% by weight, with emulsifier contents of 0.9 - 0.4% by weight, based on the monomer. Such a process is described in German Published Application DAS 1,964,029.
When discharging such high-percentage, low-emulsifier dispersions from large reactors, e.g., with volumes of 60 m.sup.3 or more, a considerable problem has evolved. For example, attempts to proceed analogously to the customary mode of operation employed in connection with small reactors, i.e., to discharge the dispersion by pumping, have failed when employed in connection with a large reactor.
Using a positive displacement pump, which is mechanically gentle to the dispersion, at most 20-30 tons of dispersion can be conveyed per hour. In such a case, it would require about 3 hours to empty a reactor of a capacity of 100 m.sup.3 and 6 hours in case of the more conventional 200 m.sup.3 reactors. Such consumption of time solely for the removal of the dispersion by pumping would unduly lengthen the preparation times between the polymerization processes and/or would considerably reduce the space-time yield of the large reactor.
However, the use of high-speed centrifugal pumps is not possible, since high-percentage, low-emulsifier dispersions do not withstand the high mechanical stresses associated therewith.
Therefore, heretofore large reactors have beem emptied by expanding, after the end of the polymerizing step, the vinyl chloride monomer which is still under pressure into the gasometer, applying a vacuum, and then forcing the high-percentage dispersion through the discharge device with nitrogen. The vinyl chloride-nitrogen mixture remaining in the reactor after the discharging step then had to be blown off into the atmosphere, since a separation for a reuse of the vinyl chloride would be too expensive.
Consequently, not inconsiderable quantities of monomeric vinyl chloride were emitted. This method thus represented simultaneously a saving loss and also pollution of the environment. Moreover, the above-described mode of operation also required a great expenditure of time in case of large reactors, since the evacuation step, including the required vacuum tests, in a large reactor always took several hours.
These disadvantages of the previous mode of operation have been overcome by a process of this invention.
It is an object of this invention to reduce vinyl chloride emissions from the gas phase of large reactors into the atmosphere between emulsion polymerizing in the reactors. It is another object to provide a faster method of discharging high solids content polyvinyl chloride emulsions from large reactors. Other objects will be apparent to those skilled in the art.